Automotive vehicle and engine manufacturers, fuel injection equipment suppliers and, indeed, society as a whole, share in the desire for efficient, effective transportation. The balance between combustion processes to produce power, and those processes which create pollution, is best addressed by enhancing the fundamental efficiency of the engine processes.
It is well known that the ideal Carnot cycle, in which isothermal heat addition and rejection are combined with isentropic compression and expansion, is the most efficient engine cycle for any given upper and lower operating temperatures. However, the Carnot cycle is not practical for an expanding chamber piston engine due to the very high (over 50:1) compression ratio required to produce significant power. Nevertheless, a practical process which could make some use of the highly efficient Carnot process would be an advance in the art.
The most practical engine, and thus presently the most predominant, is the Otto cycle engine which includes a compression process of a fuel-air mixture followed by unregulated combustion. It is well known that for a given compression ratio the ideal Otto cycle is the most efficient expanding chamber piston engine since the Otto cycle combines high peak temperature with a practical average temperature of heat input. However, the high peak combustion temperature of an Otto engine can cause auto-ignition of a portion of the fuel-air mixture, resulting in engine noise and damage to the engine, as well as the creation of excess amounts of undesired NOx.
In the past, auto-ignition in Otto cycle engines was reduced by use of chemical additives to the fuel such as lead compounds (no longer permitted by law), manganese compounds (which cause spark plug deposits to build up, resulting in misfire), benzene (the use of which is presently being curtailed by legislative mandate) or fuel reformulations to prevent deleterious auto-ignition while meeting environmental goals. Auto-ignition can also be reduced by limiting the combustion temperature, either through use of a lower compression ratio (which reduces both power and efficiency), or by exhaust gas recirculation, lean-burn or stratified charge techniques, all of which cause power loss.
For general purpose road use, the engines of emission-constrained passenger cars are presently limited to useful compression of about 10:1. Above that limit the increased cost of the fuel control system and the additional cost of more platinum or rhodium for exhaust catalytic converters generally outweighs the benefit of higher compression ratios. A technology which would allow a practical Otto compression process to operate at compression ratios higher than 10:1 would be an advance in the art.
An improvement on the Otto cycle, as represented by a higher useful compression ratio, is an ideal Diesel cycle comprising isothermal heat addition and isochoric (constant volume) heat rejection combined with isentropic compression and expansion. This ideal Diesel cycle overcomes the fuel octane limit of the Otto cycle by utilizing air alone for the compression process and mixing the fuel with the process air as part of the combustion process. This allows use of a low octane-rated fuel, but requires cetane-rated fuel (enhanced auto-ignition). However, the isothermal process of the aforedescribed ideal Diesel cycle was found to be impractical, due to the extremely high compression ratio (50:1) required, and an alternate heat addition process (isobaric or constant pressure) was put into practice.
Another variation on the ideal Diesel cycle is the ideal limited pressure cycle including combined isochoric and isobaric heat addition, and isochoric heat rejection combined with isentropic compression and expansion. This combustion process allows an engine to be operated at moderate compression ratios (14:1 to 17:1 for large open chamber engines) as well as high compression ratios (20:1 to 25:1 for small displacement engines).
While Diesel-type engines are fuel efficient, due to their high compression ratio, they tend to be heavier and lower in power than an Otto engine of the same displacement. In addition, all direct injection engines of the Diesel type suffer from an ignition lag which reduces the control and effectiveness of the combustion process. One way to overcome this ignition lag is to preheat the fuel to 1,500.degree. R. before injection. This produces hypergolic combustion upon injection, but is an impractical method due to the short service life of the injector nozzle.
Hybrid engine processes have been developed incorporating characteristics of both diesel and spark ignition engines but these have proven impractical for road use. Examples of these hybrid processes include the Texaco TCCS, the Ford PROCO, Ricardo, MAN-FM and the KHD-AD. All employ open chamber, direct injection spark ignition engines using stratified charge techniques to improve efficiency. These developmental engines suffer substantial power loss because of ignition lag, incomplete utilization of the process air and poor mixing of the fuel/air charge.
Because the limits of current technology are thus being reached, there exists a need for an internal combustion engine that will provide a better balance between power production, fuel efficiency, pollution creation and pollution control by use of a more practical combination of thermodynamic processes.